Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
EP-A 0 218 385 discloses a thermally developable light-sensitive material which has at least one light-sensitive silver halide containing layer on a support and which further contains a compound represented by the general formula (I):X—(—(J)m—F)n  (I)wherein X is the residue of the development restrainer; J is a divalent linkage; F is an immobilizing group that is capable of reducing the diffusibility of the compound of formula (I) or a silver salt or silver complex thereof during thermal development; m is 0 or 1; and n is an integer of 1 to 3. EP-A 0 218 385 discloses the following 1—phenyl—5—mercaptotetrazole compounds in which the phenyl group is substituted with a substituent containing an optionally substituted aryl group: 
EP-A 0 256 820 discloses a thermal developing light-sensitive material comprising a support and, provided thereon, photographic structural layers comprising at least one layer containing light-sensitive-silver halide, said photographic structural layer comprising a compound represented (1): Formula (1) X1-L1-A
wherein X1 represents a residual group of a photographic restrainer, L1 is a mere bonding hand or a divalent group and A is selected from the group consisting of a hydrogen atom, an amino group, a hydroxyl group, a carboxyl group or a salt thereof, a sulfo group or a salt thereof and a sulfin group or a salt thereof, and a compound represented by general formula (2): Formula (2) X2-L2-Bwherein X2 represents a residual group of a photographic restrainer, L2 is a divalent group and B is a ballast group. EP-A 0 256 820 discloses the following 1-phenyl-5-mercaptotetrazole compounds in which the phenyl group is substituted with a substituent containing an optionally substituted aryl group: 
EP-A 0 295 507 discloses a process for the production of colour images by the photographic dye diffusion process in which a first light-sensitive sheet material is imagewise exposed, at least one of said first light-sensitive sheet material and a second light-insensitive sheet material is moistened with an aqueous liquid and the two sheet materials are together heated to 50° C. to 100° C. with their coated surfaces in contact and then separated, said first sheet material containing, on a layer support, at least one light-sensitive silver halide emulsion layer and at least one non-diffusible colour providing compound which is capable of being decomposed imagewise in the process of development to release a diffusible dye and said second sheet material containing a salt of a strong organic base and a weak acid, wherein said first sheet material contains a combination of compounds corresponding to the following formulae I and II: wherein R1 denotes hydrogen, alkyl with up to 6 carbon atoms, halogen, hydroxy, alkoxy or substituents which together form a condensed benzene ring, and wherein R2 denotes a group which can be split off in the process of development of the material and R3 denotes hydrogen, halogen, alkyl with up to 4 carbon atoms, alkoxy, carboxy, carbalkoxy, carbonamido or sulphonamido. EP-A 0 295 507 also discloses the following 1-phenyl-5-mercaptotetrazole compounds in which the phenyl group is substituted with a substituent: 
EP-A 0 838 722 discloses a photothermographic material comprising (a) a reducible silver source, (b) a photocatalyst, (c) a reducing agent, (d) a binder, and (e) at least one compound of the following general formula (I): X-L1-D wherein D is an electron donative group of atoms, with the proviso that where D is a hydrazino group which is not a part of a semicarbazido group, no oxo group is substituted to the carbon atom which is directly attached to a nitrogen atom of the hydrazine, X ia a group capable of promoting adsorption to silver halide, and L1 is a valence bond or a linking group. EP-A 0 838 722 discloses the following 1-phenyl-5-mercaptotetrazole compounds in which the phenyl group is substituted with a substituent containing an optionally substituted aryl group: 
EP-A 0 218 385, EP-A 0 256 820, EP-A 0 295 507 and EP-A 0 838 722 all disclose the use of 1-phenyl-5-mercaptotetrazole compounds in which the phenyl group is substituted with a substituent containing an optionally substituted aryl group. However, the technology of substantially light-insensitive thermographic materials containing substantially light-insensitive organic silver salts is substantially different from that of photothermographic materials containing substantially light-insensitive organic silver salts, despite the fact that in both cases the image results from the reduction of organic silver salts. However, this a superficial similarity masking the fact that the realization of the species which catalyze this reduction is completely different, being image-wise exposure of photosensitive silver halide-containing photo-addressable thermally developable elements in the case of photothermographic recording materials and image-wise heating of thermosensitive elements which do not contain photosensitive silver halide in the case of thermographic recording materials. This difference in technology is further underlined by the nature of the ingredients used in the two types of materials, the most significant difference being the absence of photosensitive silver halide and spectral sensitizing agents in substantially light-insensitive thermographic recording materials, but also reflected in the different reducing agents used, stronger reducing agents being used in substantially light-insensitive thermographic recording materials, the different stabilizers, the different toning agents etc. Furthermore, the thermal development processes themselves are significantly different in that the whole material is heated at temperatures of less than 150° C. for periods of seconds (e.g. 10s) in the case of photothermographic recording materials, whereas in the case of substantially light-insensitive thermographic recording materials the materials are image-wise heated at much higher temperatures for periods of ms (e.g. 10-20 ms). Moreover, thermal development in substantially light-insensitive thermographic recording materials involves the liquid crystalline phases of the organic silver salts, whereas this is not the case in the thermal development step in the case of photothermographic recording materials even when using the same organic silver salts. Realization of a neutral image tone is a major problem in the case of substantially light-insensitive thermographic recording materials due to the very short heating times, whereas it is much less of a problem in photothermographic recording materials due to the much longer heating times.
EP-A 0 713 133 discloses a thermal imaging system consisting of (i) a donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver and (ii) a receiving element comprising on a support a receiving layer comprising a silver source, capable of being reduced by means of heat in the presence of a reducing agent, a binder and a stabiliser selected from the group consisting of benzotriazoles, heterocyclic mercaptanes, sulphinic acids, 1,3,4-triazo-indinolines, 1,3-dinitroaryl compounds, 1,2,3-triazoles, phthalic acids and phthalic acid derivatives. EP-A 0 713 133 discloses that preferred heterocyclic mercaptanes are mercaptotetrazoles corresponding to the following general formula (C): and discloses the following 1-phenyl-5-mercapto-tetrazole compounds: 
EP-A 0 901 040 discloses a substantially light-insensitive monosheet recording material comprising a support and a thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith and a binder, characterized in that said thermosensitive element further contains an unsaturated carbocyclic or heterocyclic stabilizer compound substituted with a -SA group where A is hydrogen, a counterion to compensate the negative charge of the thiolate group or a group forming a symmetrical or an asymmetrical disulfide and said recording material is capable of producing prints with a numerical gradation value defined as the quotient of the fraction (2.5-0.1)/(E2.5-E0.1) greater than 2.3, where E2.5 is the energy in Joule applied in a dot area of 87 μm×87 μm of the imaging layer that produces an optical density value of 2.5, and E0.1, is the energy in Joule applied in a dot area of the imaging layer material that produces an optical density value of 0.1. EP-A 0 901 040 discloses the following 1-phenyl-5-mercapto-tetrazole compounds: 
WO 94/16361 discloses a multilayer heat-sensitive material which comprises: a color-forming layer comprising: a color-forming amount of finely divided, solid colorless noble metal or iron salt of an organic acid distributed in a carrier composition; a color-developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a color-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the color-forming layer. Furthermore, WO 94/16361 discloses that suitable antifoggants are well-known photographic anti-foggants such as mercaptobenzotriazole, chromate, oxalate, citrate, carbonate, benzotriazole (BZT), 5-methylbenzotriazole, 5,6-dimethylbenzotriazole, 5-bromobenzotriazole, 5-chlorobenzotriazole, 5-nitro-benzotriazole, 4-nitro-6-chlorobenzotriazole, 5-nitro-6-chlorobenzotriazole, 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene, benzimidazole, 2-methylbenzimidazole, 5-nitrobenzimidazole, 1-phenyl-5-mercaptotetrazole, 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptothiazoline, 2-mercapto-4-methyl-6,6′-dimethylpyrimidine, 1-ethyl-2-mercapto-5-amino-1,3,4-triazole, 1-ethyl-5-mercapto-1,2,3,4-tetrazole, 2,5-dimercapto-1,3,4-thiodiazole, 2-mercapto-5-aminothiodiazole, dimethyldithiocarbamate, and diethyldithiocarbamate.
WO 96/10213 discloses a thermographic imaging element comprising a substrate having coated on at least one surface thereof a thermographic imaging system comprising at least one layer comprising light-insensitive organic silver salt; reducing agent for silver ion; binder; toner; and a dye which absorbs radiation in the wavelength range of 750-1100 nm, wherein said at least one layer comprising said light-insensitive organic silver salt forms an image density greater than about 1.0 when exposed to 0.10-2.0 joules/cm2 of said radiation in 0.20 to 200 microseconds. WO 96/10213 does not disclose a stabilizer against the influence of light, but mentions the optional incorporation of benzotriazole in the thermographic imaging element, but only exemplifies the incorporation of benzotriazole.
Substantially light-insensitive thermographic recording materials contain the imaging-forming components both before and after image formation and unwanted image-forming must be hindered both during storage prior to printing and in prints exposed to light on light-boxes e.g. during examination by radiologists. Furthermore, such stabilization must take place without adverse effects upon the image quality particularly the image tone. Thermographic printers are being introduced with ever higher throughputs, which require thermographic recording materials able to provide stabilization without an adverse effect on the image quality at such faster throughputs. There is therefore a need for stabilizers which fulfil these requirements.